$028D/653: Flag: Shift Keys
+-------+--------------------------------------------------------+
| Bit 2 | 1 = CTRL pressed |
| Bit 1 | 1 = CBM pressed |
| Bit 0 | 1 = Shift pressed |
+-------+--------------------------------------------------------+
Kernal-Reference:
LDA $028D : $EB48
LDY $028D : $EB2A
ORA $028D : $EAC1
STA $028D : $EA89 $EAC4
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